Decidualization of Ovarian Endometriosis During Pregnancy Mimicking Malignancy

Objectives The purpose of this series is to present deciduosis (the formation of extrauterine decidua) as one of the differential diagnoses of a malignant tumor during pregnancy. Methods Two cases are described in which pregnant patients had a pelvic tumor. The lesions, which were diagnosed in the early second trimester, consisted of complex masses with an extensive blood supply and had a sonographic appearance of a malignant tumor. The high suspicion for malignancy necessitated surgical intervention. Results During surgery, the lesions were observed to be of an ovarian origin with papillary excrescences covering their exterior. The lesions were excised and sent for histologic examination. The results showed a markedly decidualized endometriotic cyst in both cases. Conclusions This phenomenon is a diagnostic challenge and should be considered in the differential diagnosis of a malignant mass during pregnancy.